This invention relates to a method for the manufacture of a III-V compound semiconducting single crystal refined to a high degree of purity.
Of the III-V compound semiconducting single crystals, the high-purity single crystal of GaAs constitutes itself a semi-insulator having resistivity of at least 10.sup.6 .OMEGA..cm. When a device is constructed by using this GaAs crystal as a substrate for an optoelectronic integrated circuit formed of an integration of optical elements and electronic elements, it can decrease the floating capacity due to wire distribution and facilitate the separation between elements and, therefore, add greatly to the density of integration.
As the techniques available for the production of the semi-insulating GaAs, there have been heretofore known the boat growth method, the low-pressure encapsulated pulling method, and the high-pressure encapsulated pulling method. Although the boat growth method can be carried out under atmospheric pressure, it has the disadvantage that silicon from quartz as the material for the boat will pass into the semi-insulating material and lower the resistivity thereof. To compensate for the loss of resistivity, there is generally followed a practice of adding enough chromium to increase the semi-insulating property of the material. This practice, however, entails a possibility that the added chromium will be unevenly distributed and, as the result, the properties will be dispersed or these properties will be altered during the thermal process. The low-pressure encapsulated pulling method uses as the starting material the polycrystalline GaAs produced by the boat growth method. Since this starting material has a low degree of purity, this method necessitates addition of chromium. The high-pressure encapsulated pulling method which does not require any addition of chromium effects formation of the crystal under application of high pressure and, therefore, disadvantageously causes most of the produced crystals to sustain defects due to the influence of a hot atmosphere under pressure. Further, the encapsulant and the high-pressure gas convection have their effects in complicating the procedure of manufacture and impairing the uniformity of resistance distribution in the formed crystals. This method, accordingly, has found it difficult to produce single crystals having resistivity of at least 10.sup.6 .OMEGA..cm and enjoying uniform resistance distribution with satisfactory repeatability. Possible reasons for this difficulty may be that no method has yet been established for effecting desired refining of GaAs melt and that a method for determining the quality of GaAs melt has not been available.